Traditional terrestrial commercial radio broadcasters transmit an analog signal. However, use of a digital signal allows broadcasters to transmit more data in the same amount of bandwidth as an analog signal. Such digital audio broadcasting is therefore capable of providing listeners with a much higher audio quality and various additional information typically in the form of scrolling text on the radio receiver's display, such as traffic and weather reports, song titles and artist information, sports scores, and the like.
Among the standards proposed for providing digital radio service, the In-Band On-Channel (“IBOC”) transmission method has gained acceptance for its ability to allow simultaneous transmission of analog and digital signals within the same channel. By using IBOC, a radio broadcaster avoids the need for additional frequency allocations. Accordingly, a listener with a digital radio receiver may tune to customary frequencies to receive digital broadcasts. The National Radio Systems Committee (“NRSC”) promulgates an IBOC digital radio broadcasting standard, presently NRSC-5, that sets out the requirements for a system for broadcasting digital audio and ancillary digital data signals over AM and FM broadcast channels that may contain analog signals.
Broadcasters using IBOC digital radio broadcasting (commonly termed “HD Radio”) transmit digital subcarrier signals in sidebands along with analog signals, or hybrid transmissions. FM IBOC, for example, creates a set of upper and lower sidebands on each side of the analog carrier frequency. Those hybrid transmissions, including any noise and spuriously generated signals such as phase noise and intermodulation products, generally must fit within a spectral mask, for example, the spectral mask specified by the NRSC for hybrid transmissions. FIG. 1 illustrates one such spectral mask 1 for an FM hybrid transmission within which the analog carrier frequency 2 and the upper and lower digital sidebands 3 of the hybrid transmission appear.
Broadcasters rely on a number of different methods for HD Radio implementation. For FM HD Radio, for example, one such method, known as “High-Level” Combined HD Radio, involves separately creating and amplifying analog and digital signals, combining those signals and feeding them to a common antenna, such as in the system illustrated in FIG. 2. In FIG. 2, a combiner 10 (also known as an injector) may receive at the analog RF input port 11 an analog signal from an analog FM transmitter 12 and receive at the digital RF input port 13 a digital signal from a digital FM transmitter 14, combine the analog and digital signals and feed the hybrid signal to an antenna 16 through the antenna output port 15. The process of combining analog and digital signals generally results in substantial RF energy loss, which is dissipated to a reject load 18 through the reject load output port 17. For a 10 dB combiner, for example, such losses are normally at about 0.5 dB between the analog RF input port 11 and the antenna output port 15, and at about 10 dB between the digital RF input port 13 and the antenna output port 15. That is, the reject load 18 typically dissipates 90% of the digital signal and about 10% of the analog signal.
The combiner 10 may also isolate the analog and digital transmitters 12 and 14 to avoid the generation of spurious or intermodulation products. Typical combiners generally provide such isolation in the range of 30 to 45 dB. For example, according to the iBiquity Digital Corporation, an organization responsible for developing the IBOC standards promulgated by the NRSC, the minimum recommended isolation is 36 dB.
It has been found that a tube-type analog FM transmitter generally requires substantially more (at least 10 dB) isolation from a digital FM transmitter than solid-state analog FM transmitter. Approximately 30 dB of isolation from a tube-type analog FM transmitter into a solid-state digital FM transmitter, or from a tube-type digital FM transmitter into a solid-state analog FM transmitter is generally sufficient to avoid the generation of spurious or intermodulation products.
Typical combiners have been found to generally perform according to their manufacturers' specifications when all four ports, i.e., the analog RF input port, the digital RF input port, the antenna output port and the reject load output port, are terminated with precision 50Ω test loads. In that situation, the isolation characteristics of the combiner typically meet iBiquity's recommended 36 dB of isolation. However, isolation between the analog and digital RF input ports typically degrades significantly when the precision loads at the antenna output port and reject load output port are replaced with a typical FM broadcast antenna and reject load, respectively, due to the actual load impedance of the antenna and combiner imperfections. Such degradation typically reduces the isolation to below the recommended 36 dB. It has been found, for example, that the isolation from a digital solid-state FM transmitter into a tube-type analog FM transmitter is typically degraded to the point that intermodulation products produced by the analog transmitter fall outside the limits of the standard spectral mask of FIG. 1, as is illustrated in FIG. 3. In the spectral plot of FIG. 3, the intermodulation products 20 appear above and below the digital carriers 21 outside the spectral mask 22. Despite such degradation, though, the measured voltage standing wave ratio (“VSWR”) of the FM antenna over a 400 kHz bandwidth is typically less than the maximum 1.2:1 ratio recommended by iBiquity, and the measured VSWR of the reject loads is typically less than the maximum 1.1:1 to 1.15:1 ratio range recommended by iBiquity. Ideally, the characteristic impedance of the antenna is equal to the characteristic impedance of the reject load (typically at about 50Ω), and the VSWR may be used to indicate how well matched those impedances are.
It is therefore desired to improve the isolation from a digital FM transmitter into an analog FM transmitter.